1. Field of the Invention
The present invention relates to linear beam devices such as inductive output amplifiers used for amplifying an RF signal. More particularly, the invention relates to an output cavity structure for extracting an amplified RF signal from an inductive output amplifier.
2. Description of Related Art
It is well known in the art to utilize a linear beam device, such as a klystron or travelling wave tube amplifier, to generate or amplify a high frequency RF signal. Such devices generally include an electron emitting cathode and an anode spaced therefrom. The anode includes a central aperture, and by applying a high voltage potential between the cathode and anode, electrons may be drawn from the cathode surface and directed into a high power beam that passes through the anode aperture.
One class of linear beam device, referred to as an inductive output amplifier, or inductive output tube (IOT), further includes a grid disposed in the inter-electrode region defined between the cathode and anode. The electron beam may thus be density modulated by applying an RF signal to the grid relative to the cathode. After the density modulated beam is accelerated by the anode, it propagates across a gap provided downstream within the inductive output amplifier and RF fields are thereby induced into a cavity coupled to the gap. The RF fields may then be extracted from the output cavity in the form of a high power, modulated RF signal.
While inductive output amplifiers are advantageous in amplifying high frequency RF signals, such as for broadcasting television signals (e.g., 470-810 MHz tuning range with an instantaneous bandwidth of 6 MHz), the tunability within the desired range and the instantaneous bandwidth of such signals is limited by the impedance of the output cavity at the gap. To achieve wide bandwidth in klystrons, it is known in the art to use a double-tuned cavity having a tunable primary cavity which interacts with the electron beam, and a tunable secondary cavity coupled to the primary cavity. An example of a double-tuned cavity for a klystron is provided by U.S. Pat. No. 2,934,672, for "Velocity Modulation Electron Discharge Device," to Pollack et al. See also "Wide Band UHF 10 KW Klystron Amplifier," by H. Goldman, L. F. Gray and L. Pollack, IRE National Convention Record, 1958.
In the prior art double-tuned cavity disclosed by Pollack et al., the secondary cavity comprises a coaxial resonator one-half wavelength (.lambda./2) in length that is coupled to the primary cavity, where .lambda. is a wavelength of an RF output signal. An adjustable loop is disposed at one end of the coaxial resonator within the primary cavity for inductively coupling RF energy from the primary cavity to the secondary cavity. The coaxial resonator has a moveable short circuit in the secondary cavity for tuning the one-half wavelength transmission line. Energy is extracted from the coaxial resonator by a capacitative probe. Broad bandwidth operation is achieved by tuning the secondary cavity to a desired frequency range.
While the double tuned-cavity disclosed by Pollack et al. was effective for its time at relatively low power levels (e.g., around 10 KW), it is not practical for present inductive output amplifiers that are expected to operate at much higher power levels (e.g., above 30 KW). This is due in part to the relatively small circumference of the short circuit at the end of the coaxial resonator of the secondary cavity. In particular, the moveable short circuit of the secondary cavity relies upon a plurality of conductive fingers to maintain electrical contact between the circumference of the short circuit and the outer conductor of the coaxial resonator. The output current conducted through the coaxial resonator passes directly through the conductive fingers. At the high power levels expected of inductive output amplifiers, the current density may be high enough to damage the conductive fingers. It is not possible to enlarge the circumference of the short circuit to reduce the current density without altering the resonant characteristics of the coaxial resonator.
Thus, it would be desirable to provide an inductive output amplifier having a double-tuned output cavity providing a wide tuning range and an ability to handle high output current levels.